1. Field of the Invention
The present invention relates to a numerical control grinding machine of the type wherein movements of a grinding wheel head and a work table respectively in first and second directions transverse to each other are controlled by reference to the contents of first and second present position registers which respectively stores the positions of cylindrical and shoulder grinding surfaces of a grinding wheel in the first and second directions.
2. Description of the Prior Art
In numerical control grinding machines, the positions of cylindrical and shoulder grinding surfaces of a grinding wheel are gradually changed by grinding wheel dressings. Thus, not the position of a wheel head, but the positions of the grinding wheel surfaces must be precisely detected to control movements of the wheel head and a work table.
To this end, in a known numerical control grinding machine of the type mentioned above, a dressing tool is provided on a work table, and the positions of the tip of the dressing tool in first and second directions which are respectively perpendicular and parallel to a work spindle axis are measured in advance to be used as reference positions. Each time a dressing on the grinding wheel is completed, each of the present position registers is set with a corresponding one of the reference position data, with the dressing tool being engaged with a corresponding one of the grinding surfaces. This enables the contents of the present position registers to respectively represent the actual positions of the cylindrical and shoulder grinding surfaces.
However, in the known apparatus, when a power failure occurs during a machining operation, the contents of the present position registers which respectively store positions of the cylindrical and shoulder grinding surfaces are lost, and the positions of the grinding surfaces become unknown. In this event, it is necessary to perform the grinding wheel dressing manually and to set the present position registers with the reference position data determined by the dressing tool. Thus, the working property of the known apparatus is poor.
Moreover, in the known apparatus, when the position of the tip of the dressing tool is deviated from the reference position due to a wear of the dressing tool, the positions of the grinding surfaces after dressing operations are deviated from the theoretical positions. Since the present position registers are set with the predetermined reference position data even at such time, it results that the contents of the present position registers represent those deviated from the actual positions of the grinding surfaces, whereby a machining error may be involved.